1. Field of the Invention
The present invention relates to an electron gun for a cathode ray tube, and particularly, to an electron gun for a cathode ray tube that is capable of improving an image quality of a screen by optimizing a shape of an electron beam by correcting an aberration according to a deflection angle of the electron beam.
2. Description of the Background Art
Generally, a cathode ray tube, which optically implements an image by converting an electric signal into an electron beam and discharging the electron beam onto a phosphor screen, is widely used since excellent display quality is achieved at an affordable price.
As shown in FIG. 1, the cathode ray tube includes: a panel 11 of a front glass; a funnel 16 of a rear glass forming a vacuum space by being coupled with the panel 11; a phosphor screen 15 deposited on an inner surface of the panel 11 and serving as a phosphor; an electron gun 1 for emitting electron beams 13 which makes the phosphor screen 15 emit light; a deflection yoke 12 mounted on an outer circumferential surface of the funnel 16 with a predetermined interval for deflecting the electron beam 13 to the phosphor screen 15; a shadow mask 14 installed at a constant interval from the phosphor screen 15; a mask frame 18 for fixing and supporting the shadow mask 14; an inner shield 19 extending from the panel 11 to the funnel 16 for shielding external terrestrial magnetism and thus preventing deterioration of color purity by the magnetism; and a holder 17 for elastically supporting the mask frame 18 to an inner side of the panel 11.
In the conventional cathode ray tube, the electron beam 13 emitted from the electron gun 1 is deflected by the deflection yoke 12, passes through a plurality of electron beam passing holes formed at the shadow mask 14, and lands on the phosphor screen 15 deposited on the inner surface of the panel 11. Accordingly, the deflected electron beam 6 makes the phosphor formed at the phosphor screen 15 emit light, thereby achieving an image.
Hereinafter, the electron gun 1 of the conventional cathode ray tube will be described with reference to FIG. 2.
The electron gun 1 can be divided into a triode and a main lens unit according to operations.
The triode comprises a cathode 3, in which a heater 2, thermal source is built in for discharging thermal electron, and arranged in-line; a control electrode 4 for controlling thermal electron discharged from the cathode 3; and an accelerating electrode 5 for accelerating the electron beam 13. Herein, the control electrode 4 is grounded, and a low voltage of 500V˜1000V is applied to the accelerating electrode 5.
The main lens unit comprises a focusing electrode 8 for focusing the electron beam 13 emitted from the triode and an anode 9 for finally accelerating the electron beam. High voltage of 25˜35 KV is applied to the anode 9, and middle voltage about 20˜30% of the voltage applied to the anode 9 is applied to the convergent electrode 8.
Therefore, a static electron lens is formed between the anode 9 and the focusing electrode 8 due to potential difference between voltages applied to the anode 9 and to the convergent electrode 8 so that the electron beam 13 is focused toward the phosphor screen 15.
Also, the focusing electrode 8 comprises a first focusing electrode 8a adjacent to the triode and a second focusing electrode 8b adjacent to the anode 9. Further, a static voltage is applied to the first focusing electrode 8a, and dynamic voltage is applied to the second focusing electrode 8b. Therefore, a quadrupole (hereinafter, referred to as quadrupole lens) is formed between the first focusing electrode 8a and the second focusing electrode 8b. 
Meanwhile, reference numerals 6, 7 indicate focusing electrodes for focusing the electron beam 13 emitted from the triode.
Hereinafter, the quadrupole lens will be described as follows.
That is, in order to realize the image, the electron beams 13 should land on the proper areas of the phosphor screen 15, and therefore, the electrode beams 13 should be deflected to the whole area of the screen 15. Generally, since the electron beams of red, green and blue colors are arranged in parallel in the cathode ray tube using the in-line type electron gun 1, a self-convergence deflection yoke 12 using inhomogeneous electromagnetic field is used in order to focus the respective electron beams 13 on one point of the screen 15. In the distribution of the electric field generated by the self-convergence deflection yoke 12, horizontal deflection electromagnetic field is applied by a pincushion type, and vertical deflection electromagnetic field is applied by a barrel type as shown in FIGS. 3A and 3B. Therefore, as shown in FIGS. 4A and 4B, there are dipolar component and quadrupolar component. The dipolar component deflects the electron beam toward horizontal and vertical directions, and the quadrupolar component converges the electron beam in the vertical direction and diverges in the horizontal direction, and therefore, the beam in vertical direction is converged with shorter distance than that of the horizontal direction to cause a halo phenomenon that the electron beam is risen bulgingly in the vertical direction on periphery of the screen. That is, as shown in FIG. 5, since the deflected electric field of the deflection yoke is not applied on the center portion of the screen 15, electron beam spot has an exact shape. However, the deflected electric field of the deflection yoke 12 is applied on the periphery of the screen 15, and therefore, the electron beam 13 is diverged in the horizontal direction and converged in the vertical direction. Therefore, the shape of the electron beam spot is formed as a horizontally elongated core shape of high density in the horizontal direction, and a halo, which is an inflected form of low density, is generated in the vertical direction to cause the inferiority of the screen resolution on the periphery of the screen. These problems become worse as the cathode ray tube grows larger and the deflection angle of the electron beam becomes larger.
Therefore, in order to solve the above problems, the quadrupolar lens is formed between the first focusing electrode 8a and the second focusing electrode 8b as shown in FIG. 6 to compensate with the quadrupolar component generated from the deflection yoke 12, and thereby, the electron beam components of the horizontal and the vertical directions can be focused on one point at the same time. However, the electron beam 13 is focused before reaching to the screen 15 due to the difference between the distance from the electron gun 1 to the center of the screen 15 and the distance from the electron gun 1 to the periphery of the screen 15, and the halo phenomenon is still generated. Therefore, in order to improve these problems, a dynamic voltage synchronized with the deflection signal of the deflection yoke 12 is applied in order to reduce the lens magnification of the main lens, and therefore, a focal length of the electron beam is reduced to compensate aberration of the main lens when the electron beam is deflected toward the periphery of the screen 15.
However, according to the conventional dynamic focus electron gun applying the quadrupolar lens generated by applying the dynamic voltage to the electrode, very high dynamic voltage is required in order to compensate entirely the halo phenomenon of the electron beam on the periphery of the screen. In addition, in case that the electron beam is deflected to the periphery of the screen, the vertical size of the electron beam spot becomes too small and the horizontal size of the spot becomes relatively large. Therefore, a moire phenomenon that the shape of the electron beam spot is shown as a waveform is generated on the screen, and consequently the screen resolution of the periphery of the screen is lowered.